A Republican congressional candidate in a reliably blue California district managed to capture nearly a quarter of votes cast in the state’s open primary last month — just after the state Republican Party caught wind of his anti-Semitic comments and rescinded its automatic endorsement.

The candidate, John Fitzgerald, urged people on his campaign website to pay attention to “Jewish supremacism,” among other anti-Semitic views, which led party leaders to rescind their support in May, about two months after the official endorsement.

In the weeks since, Mr. Fitzgerald has increased the frequency of his anti-Semitic statements and has appeared on podcasts in which he claimed the Holocaust was a fabrication.

“Everything we’ve been told about the Holocaust is a lie,” Mr. Fitzgerald said last week on a radio show hosted by Andrew Carrington Hitchcock, an anti-Semitic commentator who has glorified Hitler.

“My entire campaign, for the most part, is about exposing this lie,” Mr. Fitzgerald said.

The brief endorsement of a Holocaust denier by a major political party in California has prompted Republican leaders there to take candidate vetting more seriously, and comes at a time when extremist and anti-Semitic candidates are receiving increased exposure on the national political stage.

Mr. Fitzgerald received 23 percent of the vote to finish second in the 11th Congressional District’s June primary, which is open to all candidates regardless of party and allows the top two finishers to qualify for the general election. He is running against Representative Mark DeSaulnier, a Democrat, in a district northeast of San Francisco that has not elected a Republican to the House since 2004.

The state’s Republican Party automatically endorsed Mr. Fitzgerald in March because party rules say the organization will automatically back the only Republican in the field, said Matt Fleming, a spokesman for the California Republican Party. The party reversed its decision in May and issued a statement denouncing his candidacy.

“Once we learned of Mr. Fitzgerald’s anti-Semitic worldview in late May, we moved immediately to undo the unfortunate automatic endorsement,” Mr. Fleming said in an email.

In an interview on Thursday, Mr. Fitzgerald said that he was not surprised the Republican Party had disavowed his candidacy because both major political parties in America are run by “Jewish elitists.”

Mr. Fitzgerald, 54, said he was a small-business owner but would not disclose what kind of business he runs. A LinkedIn page for a John Fitzgerald that links to the candidate’s campaign website lists the page’s user as a painting contractor.

The comments that alerted the party to Mr. Fitzgerald’s views were in a post on his campaign website asserting that Jewish people played a “prominent role” in the Atlantic slave trade and urging awareness of “Jewish supremacism.” Scholars have countered historical claims that Jewish people dominated the slave trade with research showing that their role was marginal.

Mr. Fleming said the initial endorsement of Mr. Fitzgerald was preceded by minimal vetting of his views. Now, he said, the party is bolstering its vetting process by thoroughly surveying candidates’ public comments and the content of their campaign websites.

Mr. Fitzgerald’s qualification for the state’s general election makes him the latest in a series of high-profile extremist candidates across the nation — including Arthur Jones, a Holocaust denier described as a Nazi by the Illinois Republican Party. Mr. Jones won the Republican congressional primary in March in a heavily Democratic district that includes part of Chicago. In Wisconsin, a white nationalist and anti-Semite, Paul Nehlen, is running for Speaker Paul D. Ryan’s vacated House seat.

In the Thursday interview, Mr. Fitzgerald said that the Holocaust was a “complete fabrication” and that the Israeli government was behind the Sept. 11 attacks.

“There are a few other people being honest in this country, and they’re all being lambasted for telling the truth,” Mr. Fitzgerald said, referring to Mr. Jones and Mr. Nehlen.

Mr. Fitzgerald ran for Congress in California as a Democrat in 2010 and 2012, but did not receive the party’s endorsement in either case. He is running as a Republican in November but said he identifies more as an independent.

“Am I a Republican?” Mr. Fitzgerald said. “What is a Republican anymore?”

Mr. DeSaulnier said in an interview on Thursday that he believed California voters — more than 36,000 of whom voted for Mr. Fitzgerald — were unaware of Mr. Fitzgerald’s views when they went to the polls. “People see ‘R’ and they see ‘D’ and that’s how they vote,” he said.

Mr. DeSaulnier, who was elected to office in 2014 and received 68 percent of the vote in the June primary, said anti-Semitism is an “ugly and evil” set of beliefs that he does not believe the residents in his district will ultimately support.

On a podcast called “The Realist Report,” released last month and hosted by John Friend — a self-described independent journalist whom the Anti-Defamation League has called an anti-Semite — Mr. Fitzgerald lamented “Jewish control and supremacy.” In a blog post on his website, Mr. Fitzgerald wrote that Jewish people are behind a push for “multiculturalism, diversity and inclusiveness” throughout the United States and other “once predominantly white nations.”

Mr. Fleming, the state Republican Party spokesman, said that if only one Republican joins the race in California’s open primary system — often called the “jungle primary” — the state party’s bylaws say the organization will automatically endorse that candidate. He said that before the California Republican Party endorsed Mr. Fitzgerald in late March, one of the few “red flags” on his campaign website was conspiracy theories about the Sept. 11 attacks. Mr. Fleming said he would have alerted state Republican leaders if he had seen those statements before the endorsement process.

Ron Nehring, a former chairman of the California Republican Party and spokesman for Senator Ted Cruz’s 2016 presidential campaign, said he was the first person to alert the organization to the views expressed on Mr. Fitzgerald’s campaign website, prompting discussion about disavowing the candidate. Mr. Nehring said he planned to propose a change to the state Republican Party rules to eliminate automatic endorsements.

Mr. Fleming said the transition to the “top-two” primary system, adopted by voters in 2010, has required an adjustment period for the endorsement process.

The local Republican Party in Contra Costa, a county encompassing the 11th District, never endorsed Mr. Fitzgerald because he did not attend any of their meetings or request an endorsement, said Matt Shupe, the chairman of the organization. Mr. Fitzgerald said he had been unsuccessful in trying to reach local Republican leaders.

Mr. Shupe called Mr. Fitzgerald’s views unwelcome in the party, but said there was “no legal way” to prevent a person from getting on the ballot.

Mr. Fitzgerald denied that his views were anti-Semitic.

“I have friends that are Jewish,” Mr. Fitzgerald said. “I have no issue with any people. I have issues with people who lie. It’s the elitists who control it all.”